In wireless communication systems, an air interface is used for the exchange of information between a mobile (e.g., cell phone) and a base station or other communication system equipment. The air interface typically comprises a plurality of communication channels. As is well known, CDMA-based systems employ unique codes for multiplexing separate, simultaneous transmissions over a communication channel. In a CDMA-based system, transmission duration is fixed while the users share the resources in the code domain. In the well-known High Speed Downlink Packet Access (HSDPA) specification in the Universal Mobile Telecommunication System (UMTS) standard, for example, the available resources are shared between circuit switched voice and data users within a standard five 5 MHz channel bandwidth. HSDPA employs 2 millisecond fixed frames comprising three time slots of equal duration, e.g., 0.67 milliseconds. Due to the real-time nature of the voice traffic, the resources (e.g., power, codes, etc.) are first allocated to voice. After satisfying the needs of real-time services, the remaining resources are then shared among the data users preferably in a time-multiplexed fashion on a frame by frame basis. HSDPA users can also be code multiplexed within a frame to fully utilize the resources in the frame, e.g., if a single user cannot fully utilize the resources in a frame.
The quality of any one of the communication channels can vary depending on factors such as user location, user speed, interference from other cells, and so on. For example, a particular channel between a base station and a mobile may have an acceptable throughput at one instant in time and unacceptable throughput at another instant in time. As such, the information transmitted through a relatively low quality communication channel can be adversely affected to such an extent that the information contains errors when received. In wireless communication systems, information is channel coded to compensate for errors arising during transmission. However, channel coding alone is not always sufficient to compensate for errors. Accordingly, systems employ various link layer recovery protocols, including retransmission methods, to compensate for these errors.
One widely used retransmission method is Automatic Repeat Request (ARQ). ARQ is a method of confirming that information transmitted through a communication channel has been received without any errors. Receiving equipment sends a message to transmitting equipment acknowledging that the transmitted information was received without errors. If the transmitted information was received with errors, then the receiving equipment sends a message to request a retransmission. The transmitter can retransmit all or part of the previously transmitted information using the same or different channel coding. As is well known, incremental redundancy (IR) transmission and soft combining (also referred to as Hybrid ARQ) are used to improve the efficiency of ARQ. For example, when a lost data frame is retransmitted, the receiving equipment may combine the multiple received copies of the frame to increase the likelihood of correct decoding. Alternately, the transmitter may transmit additional parity information instead of retransmitting another copy of the lost frame.
One method for incremental redundancy operation where the transmission rates can be different for different incremental redundancy transmissions was disclosed in U.S. patent application Ser. No. 09/725,438, entitled “Rate Adaptation in a Wireless Communication System” and filed Nov. 29, 2000, the subject matter of which is incorporated herein by reference in its entirety. As described, the different transmission rates are obtained by varying the transmission time while keeping the information block size unchanged. The aforementioned reference also described a scheme in which the retransmissions only provide the additional redundancy needed to successfully decode the frame based on the quality of the previous incremental redundancy transmissions. For example, if the first transmission is performed over four (4) time slots, the retransmission can be performed over two (2) slots if the quality of the previous transmission required only two (2) slot transmission for successful decoding of the information block.
Disadvantageously, the adaptive incremental redundancy scheme in the prior art requires variable length transmissions that lead to a more complex system in terms of implementation and control signaling transmission. Moreover, the granularity that is provided by the time slot duration for transmission of redundancy in most practical systems is coarse and therefore does not promote an efficient use of the bandwidth.